Percutaneous absorption of xenobiotics across skin is a process governed by diffusion, with the nonviable stratum corneum as the rate-limiting barrier. The epidermis and dermis are viable and contain the mixed function oxidase system (cytochrome P-450), which is responsible for the metabolism of hormones and detoxification of xenobiotics. The interaction of these two processes, simultaneous absorption and metabolism of compounds in the skin in vivo, are critical to understanding the pharmacological efficacy and fate of compounds topically applied. Although previous studies have investigated the influence of dose, hydration, stratum corneum thickness and vehicle on percutaneous absorption across human skin, there is a paucity of data concerning the influence of these parameters on skin metabolism and the interaction of these two processes in vivo. The human skin sandwich flap (HSSF) we developed is uniquely qualified to study absorption and metabolism in human skin, because the flap has intact functional, viable human skin, which is maintained by a biological support system with a single isolated, yet accessible artery and vein. Simultaneous absorption and metabolic events occurring in the skin can therefore be monitored by sampling blood from the vein immediately draining the area of skin on the flap exposed to drug. Local skin metabolism can be compared and contrasted with systemic events by analyzing local and systemic blood samples, urine, and skin biopsies. The broad objective of this proposal is to define the interactive processes of percutaneous absorption and metabolism of topically applied substances in skin. The proposed research focuses on experiments which are directed at two major hypotheses 1) the flap can be utilized for studying cutaneous metabolism and 2) physiological and physical parameters can be manipulated to alter cutaneous metabolism. Specific aims include the development of analytical techniques to identify and quantify cutaneous metabolism; comparison of skin and hepatic metabolism; comparison of metabolism in grafted and nongrafted skin; investigating the disposition of parent and metabolites in blood, urine and skin; study of the influence of dose, stratum corneum thickness, blood flow and cyclosporine on cutaneous metabolism. Understanding the interaction between absorption and metabolism processes in the skin are imperative to furthering our knowledge of skin pharmacology, homeostasis, protection from toxic compound exposure, bioequivalency and immunology of skin irritation and allergic reaction.